Induction heating occurs in electrically conducting material when such material is placed in a time-varying magnetic field generated by an alternating current (ac) flowing in an induction heating coil. Eddy currents induced in the material create a source of heat in the material itself.
Induction heating can also be used to heat or melt non-electrically conducting materials, such as silicon-based, non-electrically conductive fibers. Since significant eddy currents cannot be induced in non-electrically conductive materials, they cannot be heated or melted directly by induction. However, the non-electrically conductive material can be placed within an electrically conductive enclosure defined as a susceptor. One type of susceptor is a cylinder through which the non-electrically conductive material can be passed. In a manner similar to an induction coil disposed around the refractory crucible of an induction furnace, an induction coil can be placed around a susceptor so that the electromagnetic field generated by the coil will pass through the susceptor. Unlike a refractory crucible, the susceptor is electrically conductive. A typical material for a susceptor is graphite, which is both electrically conductive and able to withstand very high temperatures. Since the susceptor is electrically conductive, an induction coil can induce significant eddy currents in the susceptor. The eddy currents will heat the susceptor and, by thermal conduction or radiation, the susceptor can be used to heat an electrically non-conductive workpiece placed within or near it.
In many industrial applications of induction heating of non-electrically conductive materials such as artificial materials and silicon, it is often desired to provide a predetermined and controlled temperature distribution along the length of the susceptor to control the heat transfer to the electrically non-conductive workpiece place within it. This can be accomplished by the delivery of different densities of induction power to multiple sections of the susceptor along its length.
The susceptor can be surrounded with multiple induction coils along its length. Each coil, surrounding a longitudinal segment of the susceptor, could be connected to a separate high frequency ac power source set to a predetermined output level. The susceptor would be heated by induction to a longitudinal temperature distribution determined by the amount of current supplied by each power source to each coil. A disadvantage of this approach is that segments of the susceptor located between adjacent coils can overheat due to the additive induction heating effect of the two adjacent coils. Consequently, the ability to control the temperature distribution through these segments of the susceptor is limited.
Alternatively, the multiple coils could be connected to a single high frequency ac power source for different time intervals via a controlled switching system. Since high electrical potentials can exist between the ends of two adjacent coils when using a single power supply, it may not be possible to locate the ends of the coils sufficiently close to each other to avoid insufficient heating in the segment of the susceptor between the ends of the coil without the increased risk of arcing between adjacent coil ends. Consequently, this approach also limits the ability to control the temperature distribution through these segments of the susceptor.
There is a need for a heating device having an induction coil in which the turns of adjacent coil sections allow induction power to be delivered in a controlled manner to preselected sections along the length of the susceptor and, consequently, to a workpiece placed within or near the susceptor, including segments between coil sections, thus eliminating cold or hot spots and permitting a desired preselected temperature distribution along the length of the susceptor. This will permit a non-electrically conductive workpiece placed within the susceptor to be heated at the preselected temperature distribution by thermal conduction and radiation.
The present invention fills that need.